In general, a semiconductor wafer is subject to a rough polishing (primary polishing), then a finish polishing (secondary polishing), and thereafter a device processing. The finish polishing is performed using a slurry that contains colloidal silica to obtain an ultramicroscopic surface roughness on the surface. The contained colloidal silica is extremely small to have a diameter of several tens nm by the spherical approximation. Since such slurry containing colloidal silica is more costly, it is considered that recycled slurry or diluted slurry may be used.
Here, the colloidal silica is a compound of amorphous anhydrous silicic acid in a colloidal state and may include unmodified colloidal silica as well as modified colloidal silica, surface of which is modified with ions or compounds such as ammonia, calcium, alumina, and so on so that colloidal particles thereof have modified ionic properties and behavior in response to a pH change. The colloidal silica may also include ultrahigh purity colloidal silica prepared by a sol-gel method and may also refer to a dispersion fluid having silica particles of colloidal sizes dispersed in water or organic solvent. In general, a slurry refers to a suspension that is also called a slip or a slime and may include a mixture having minerals, sludge, and so on dispersed in liquid. The slurry may be a highly viscous (pulpy) fluid substance. In particular, the slurry may include a chemical solution containing abrasive grains used for CMP (chemical mechanical polishing) or wafer lapping.
As an example of recycling of the slurry, art to reproduce a CMP slurry, which has a sufficiently low density of coarse particles such that a semiconductor wafer can be polished with the CMP slurry without causing deep scratches, from a waste liquid of the used CMP slurry is disclosed. In this example, a removal step of removing the coarse particles in the CMP slurry having been used for polishing so as to reduce the number of coarse particles in the waste liquid, and a concentrating step of applying a centrifugal force to the waste liquid after the removal step to concentrate the waste liquid so as to obtain a CMP slurry raw material are performed. Thus, the method of manufacturing a CMP slurry raw material, in which the CMP slurry raw material is reproduced from the recycled waste fluid of the used CMP slurry, is disclosed (for example, Japanese Unexamined Patent Publication 2002-170793).
As another example of recycling a used slurry, a technology aiming to ultimately utilize a recycled polishing slurry without any problems is disclosed (for example, Japanese Unexamined Patent Publication 2004-75859) as the used slurry such as a CMP (chemical mechanical polishing) slurry is purified by removing metal ions therefrom such that metal contamination of semiconductor wafer and the like is prevented as much as possible. In this technology, a method of purifying the slurry is provided as chelate-forming fibers, in which a functional group having a metal-chelate-forming ability is introduced into a fiber molecule, can efficiently capture and remove metal ions of iron, aluminum, copper, nickel, zinc, chromium, molybdenum, tungsten, etc. existing in the polishing slurry.
Furthermore, a technology aiming to remove aggregated abrasive grains, cutting debris, and other unwanted matter without using a filter in recycling a used slurry is disclosed (for example, Japanese Unexamined Patent Publication 2004-63858). In this technology, the used slurry discharged from a CMP apparatus or other polishing apparatuses is subject to a concentration adjusting process, a particle diameter adjusting process, and a pH adjusting process. Here, the particle diameter adjusting process is characterized in that the processing is performed by a particle diameter adjustment process unit comprising an aggregated abrasive grain pulverization process unit that performs pulverization by an ultrasonic wave irradiation process or the like; a temperature separation process unit for separating aggregated abrasive grains from normal abrasive grains by the control so as to keep the slurry in a non-uniform temperature; and an aggregated abrasive grain discharging process unit for discharging the aggregated abrasive grains and the like having been separated.
Also, a technology aiming to provide a recovery apparatus of a polishing material for recovering and recycling particles of the polishing material efficiently from a waste fluid containing the polishing material discharged from CMP processing adopted in a semiconductor manufacturing plant or the like is disclosed (for example, Japanese Unexamined Patent Publication 2002-331456). Here, as the recovery apparatus of the polishing material which is recovered from the waste fluid of the CMP processing with a silica-based slurry, an apparatus comprising a membrane separation unit into which the waste fluid is introduced, a cleaning unit for cleaning the concentrated fluid obtained by the membrane separation unit, and an adjusting unit for adjusting the pH of the concentrated fluid having been cleaned is disclosed.
Resource saving and cost reduction may be achieved in the above-described slurry recycling methods and the like. However, the polishing characteristics of such recycled slurry do not excel those of an unused slurry, and the recycled slurry can be evaluated as a substitute for the unused slurry. Therefore, it is not necessarily possible to manufacture a slurry having better polishing characteristics with such methods.
Meanwhile, a technology aiming to provide an aqueous dispersion for chemical mechanical polishing capable of sufficiently flattening a surface having been polished and having high storage stability is disclosed (for example, Japanese Unexamined Patent Publication 2004-266155), in which the aqueous dispersion for chemical mechanical polishing is prepared by mixing an aqueous dispersion (I) that is obtained by blending at least a water-soluble quaternary ammonium salt, an inorganic acid salt, and an aqueous medium; and an aqueous dispersion (II) that is obtained by blending at least a water-soluble polymer, a basic organic compound excluding a water-soluble quaternary ammonium salt, and an aqueous medium, and further combining abrasive grains with at least one of the aqueous dispersions (I) and (II). In this chemical mechanical polishing method, surface defects such as dishing, erosion, and scratch in the processing of flattening the surface having been polished can be suppressed, and polishing removal selectivity between polysilicon and silicon oxide and polishing removal selectivity between polysilicon and nitrides are evaluated high. It is also disclosed that the aqueous dispersion for chemical mechanical polishing has high stability in a concentrated state and exhibits excellent polishing characteristics when diluted with water.